The alkylation of an aromatic ring by alkyl halides, alcohols, and olefins is an extremely useful and important reaction. Commercial processes are replete with examples of aromatic alkylation to afford a diversity of products. Where the aromatic ring being alkylated is that of a diarylamine, the alkylation products, and especially the dialkylated diarylamine-i.e., the product where each aryl moiety of a diarylamine is monoalkylated-find substantial use in, for example, automotive products such as transmission fluids and power steering oils. The traditional catalyst employed for the latter alkylations are for the most part traditional Friedel Crafts catalysts such as aluminum chloride, boron trifluoride, and so forth. However, other catalysts, and in particular solid catalysts such as zeolites, synthetic molecular sieves, and silica-alumina, have been extensively employed as alkylation catalysts, especially where a bed of solid catalyst is desired to be utilized in a continuous alkylation process.
As is often the case there is not just a single reaction occurring during the alkylation of a diarylamine by an olefin, but instead one finds several concurrent reactions which tend to afford undesirable byproducts, to lower the conversion of the diarylamine to the sought-for products, to consume olefin unproductively, and often to substantially complicate the isolation of the desired reaction products because of the increased complexity of the reaction product mixture. More particularly, with olefins as alkylating agents alkylation catalysts also cause oligomerization of the olefins, cracking of the olefins, and cracking of the alkyl moiety of the alkylated diarylamine, all of which are acid catalyzed reactions. This can be illustrated using diisobutylene as the representative olefin and diphenylamine as the representative diarylamine.
__________________________________________________________________________ Diisobutylene.fwdarw. 2 isobutylene CRACKING Diisobutylene.fwdarw. dimers, trimers, etc. OLIGOMERIZATION (C.sub.6 H.sub.5).sub.2 N + diisobutylene.fwdarw. C.sub.8 H.sub.17 --C.sub.6 H.sub.4 --NH--C.sub.6 H.sub.4 --C.sub.8 H.sub.17 ALKYLATION C.sub.8 H.sub.17 --C.sub.6 H.sub.4 --N--C.sub.6 H.sub.4 --C.sub.8 H.sub.17 .fwdarw. C.sub.8 H.sub.17 --C.sub.6 H.sub.4 --N--C.sub.6 H.sub.4 --C.sub.4 H.sub.9 + isobutylene CRACKING __________________________________________________________________________
Our interest was to identify a solid catalyst usable in a fixed bed continuous process which would effect the alkylation of diarylamines by olefins: 1) at temperatures no greater than 225.degree. C; 2) with high (at least 95%) conversion of the diarylamine; 3) at an olefin to diarylamine ratio of no more than 10:1; 4) with good selectivity to the dialkylated product; 5) with excellent (&gt;90%) selectivity to alkylated products where the alkyl group has the same carbon numbers as the olefin, and; 6) with minimal oligomerization and cracking of the olefin. An additional critical requirement is that the catalyst must be regenerable. Each of the foregoing conditions can be examined individually for its benefits. Lower temperatures are desirable not only to minimize energy costs but also to minimize thermal reactions which generally are degradative. A high conversion of the diarylamine is synonymous with its efficient utilization, so long as the conversion is attended by high selectivity to the desired products. A lower olefin to diarylamine ratios means that less olefin needs to be recycled and therefore smaller reactors are needed for any given productivity. A good selectively to dialkylated product is important where the latter is the product sought, and a high selectivity to retaining the carbon number of the olefin means that one is avoiding the formation of products other than what is sought. Finally, minimization of oligomerization and cracking of the olefins means minimizing olefin loss.
The family of catalysts which we have identified that satisfy all of the above criteria are pillared clays whose pillars have been modified by incorporation of rare earth salts. More specifically, the catalysts are clays intercalated with pillars of oligomeric oxycations of aluminum, zirconium, or chromium which are complexed with or otherwise incorporate rare earth cations. These materials are described in great detail in U.S. Pat. Nos. 4,952,544 and 4,957,889, both of which are hereby incorporated by reference. For convenience, this class of materials subsequently will be referred to within as rare earth pillared clays and will be described more fully within.